The goal of the proposed study is to establish whether there is a cellular basis for functional interactions between septohippocampal neurons containing acetylcholine and GABA and neurons containing other peptidergic and monoaminergic transmitters in the hippocampal formation and septal complex. This pathway and transmitters were chosen for study based on extensive physiological and pharmacological evidence for their role in learning and memory. Four specific studies are proposed which will employ electron microscopic dual labeling techniques for localizing (1) two antigens on single sections of rat brain using combinations of immunoperoxidase, immunogold and immunoautoradiographic labels and (2) single antigens using immunocytochemical labels combined with the histochemical demonstration of transport horseradish peroxidase or plant lectins for the identification of neuronal pathways. Study I will examine septal afferent terminals in the hippocampal formation with respect to coexistence and/or synaptic relations with cholinergic or GAB terminals. Studies II and III will determine the synaptic relations between cholinergic and GABAergic terminals in the hippocampal formation. In all three studies the synaptic relations of these terminals first will be examined with respect to all types of hippocampal neurons and subsequently with respect to known intrinsic transmitters such as GABA and the neuropeptides, somatostatin and neuropeptide Y. Moreover, septal afferent, cholinergic and GABAergic terminals will be examined in the hippocampal formation with respect to other subcortical afferents from monoaminergic neurons identified by antibodies to 5-hydroxytryptamine and the catecholamine synthesizing enzyme, tyrosine hydroxylase. Study IV proposes to study the synaptic relations between monoaminergic afferents and (1) septohippocampal neurons identified by retrograde transport of horseradish peroxidase following injections into the hippocampal formation, or (2) septal neurons identified by immunoreactivity for choline acetyltransferase or GABA. The results from the four studies should be complimentary and broaden the knowledge of interactions of the cholinergic and GABAergic components of the septohippocampal pathway in relation to (a) monoaminergic afferents in the septal complex and hippocampal formation and (b) neurons containing GABA, somatostatin and neuropeptide Y in the hippocampal formation. The studies will be carried out in adult male rats, but should provide basic information on the neuronal circuitry within regions implicated in specific memory disorders in man.